1. Field of the Invention
The present invention relates generally to a crankshaft supporting structure for an internal combustion engine, which provides for a decrease of vibrations therein.
2. Description of Prior Art
A crankshaft supporting structure for an internal combustion engine has been known, for instance as disclosed in Laid-open Japanese Utility Model Application No. 57-127832 (1982), in which bearing caps are integrally connected by a beam member extending along a crankshaft. Also, in Laid-open Japanese Utility Model Application No. 59-110359 (1984), there is disclosed a frame member bridging both skirt portions located on opposite sides of a main bearings in a cylinder block. This frame member is fastened with the main bearing caps by bolts. Further, in Laid-open Japanese Utility Model Application 56-25041 (1981), there is disclosed a lower block which is formed by integrally connecting bearing caps.
These prior art references can increase stiffness of the support structure around the bearing beams. Through recent studies, however, it has been found that it is difficult to reduce the vibration and the torsion of bearing caps in a back-and-forth direction without increasing the thickness of the beam of the frame member in the structure of Laid-open Japanese Utility Model Application No. 57-127832 (1982). To increase the thickness of the beam of the frame results in increasing the weight of the engine.
Furthermore, it has been found that it is not desirable to manufacture the bearing cap and lower block integrally, like a structure of Laid-open Japanese Utility Model Application 56-25041 (1981), in suppressing noises, because the resonance frequency of the unit becomes lower, and echo like sounds in the range 200-500 Hz occur.
Still further, it is difficult to obtain a sufficient stiffness to prevent the deformation movement between the bearing caps and the lower block if the construction of Laid-open Japanese Utility Model Application No. 59-110359 (1984) is adopted.
Accordingly, it was very difficult to provide an integrated bearing cap unit which can accomplish both the opposite purposes of obtaining a light weight and maintaining an adequate resonance frequency.